Field of Invention
The present invention relates generally to determining a concentration of an analyte in a medium of a living animal using a system including a sensor. Specifically, the present invention relates to a sensor that utilizes multiple sets of light sources, photodetectors, and grafts containing one or more fluorescent indicators to improve measurement accuracy by separately stimulating the indicators and quantifying light in one or more wavebands.
Discussion of the Background
A sensor may be implanted within a living animal (e.g., a human) and used to measure the concentration of an analyte in a medium (e.g., interstitial fluid (ISF), blood, or intraperitoneal fluid) within the living animal. The sensor may include a light source (e.g., a light-emitting diode (LED) or other light emitting element), indicator molecules in a graft, and a photodetector (e.g., a photodiode, phototransistor, photoresistor or other photosensitive element). Examples of implantable sensors employing indicator molecules to measure the concentration of an analyte are described in U.S. Pat. Nos. 5,517,313 and 5,512,246, which are incorporated herein by reference in their entirety.
The sensor may include a signal photodetector and a reference photodetector with associated filters. One filter may allow light emitted (e.g., fluoresced) by the indicator molecules to pass through so that the signal photodetector detects the amount of light emitted by the indicator molecules. Another filter may allow reference light having the wavelength of the excitation light emitted by the light source to pass through so that the reference photodetector detects an amount of reference light.
The light source may generate light within a small spectrum, the spectrum may overlap with the spectrum of light emitted by the indicator molecules. In other words, the light emitted by the indicator molecules and the light emitted by the light source are not independent of one another, and the light emitted by the light source may directly affect the detection of the light emitted by the indicator molecules.
Furthermore, the wavelength of the light that is absorbed by the indicator molecules for excitation may be close to the wavelength of the light emitted by the indicator molecules, and, therefore, it may be difficult to build filters that isolate the excitation light and not the light emitted by the indicator molecules (and vice versa). In fact, as no filter design is perfect for filtering light in narrow bands, there is always some cross talk or some light leakage causing undesirable effects. Because the wavelengths are similar, distinguishing between the two types of light can be difficult.
The signal photodetector and the reference photodetector may each receive both excitation light and light emitted by the indicator molecules. For instance, when the reference photodetector detects an increase in excitation light, a corresponding increase may be detected by the signal photodetector. Similarly, when the signal photodetector detects an increase in light emitted by the indicator, a corresponding increase may be detected by the reference photodetector. When this occurs, it may be difficult to accurately quantify the light coming from the indicator.
Also, the fluorescence lifetime of the indicator molecules may be short (e.g., less than a few microseconds). As a result, it may not be possible to detect the light emitted by the indicator molecules after extinguishing the light source and with no optical interference from the excitation light, as may be possible with indicator molecules having a longer fluorescence lifetime (e.g., a few milliseconds).
There is presently a need in the art for a more accurate sensor capable of determining analyte concentration in a medium of a living animal.